U.S. patent number 4,594,705 [Application Number 06/594,896] was granted by the patent office on 1986-06-10 for bus-configured local area network with data exchange capability.
This patent grant is currently assigned to Tokyo Shibaura Denki Kabushiki Kaisha. Invention is credited to Hiroshi Kobayashi, Haruki Yahata.
United States Patent |
4,594,705 |
Yahata , et al. |
June 10, 1986 |
Bus-configured local area network with data exchange capability
Abstract
A local area network in which a plurality of local equipments,
each having one or more terminals connected thereto, are connected
to different points of a common signal transmission path leading
from a central equipment. The central equipment sends out data
signals addressed to the local equipments to the common path on a
time division basis. Each local equipment sends a data signal from
the terminals to the central equipment in response to the reception
of a self-addressed data signal from the central equipment. To
avoid a collision or overspace between the data signals transmitted
from local equipments the central equipment sends out control
signals addressed to the local equipments in response to signals
from the local equipments. Each local equipment controls the
transmission start timing of the data signal to the central
equipment in response to the self-addressed control signal.
Inventors: |
Yahata; Haruki (Fujisawa,
JP), Kobayashi; Hiroshi (Tokyo, JP) |
Assignee: |
Tokyo Shibaura Denki Kabushiki
Kaisha (Kawasaki, JP)
|
Family
ID: |
26394471 |
Appl.
No.: |
06/594,896 |
Filed: |
March 29, 1984 |
Foreign Application Priority Data
|
|
|
|
|
Mar 31, 1983 [JP] |
|
|
58-53763 |
Mar 31, 1983 [JP] |
|
|
58-53764 |
|
Current U.S.
Class: |
370/257;
370/503 |
Current CPC
Class: |
G06F
13/372 (20130101); H04Q 11/04 (20130101); H04L
12/28 (20130101) |
Current International
Class: |
G06F
13/372 (20060101); G06F 13/36 (20060101); H04Q
11/04 (20060101); H04L 12/28 (20060101); H04Q
011/04 (); H04J 003/02 (); H04J 001/16 (); H04J
003/14 () |
Field of
Search: |
;370/67,13,90,94,60,96,85,86,88,56,95,100 ;340/825.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0003849 |
|
Feb 1979 |
|
EP |
|
0005045 |
|
Apr 1979 |
|
EP |
|
2526249 |
|
Nov 1983 |
|
FR |
|
2526250 |
|
Nov 1983 |
|
FR |
|
Other References
Digest of Papers Compcon 82, 24th IEEE Computer Society
International Conference (Feb. 22-25, 1982), I. Kong et al.,
"Cable-Net: A Local Area Network Reservation Scheme," pp. 182-186.
.
IEEE Electro, vol. 7, (May 1982), I. Kong. "Local Area Network--A
Broadband Implementation," pp. 1-7..
|
Primary Examiner: Olms; Douglas W.
Assistant Examiner: Chin; Wellington
Attorney, Agent or Firm: Oblon, Fisher, Spivak, McClelland
& Maier
Claims
We claim:
1. A bus-shaped local area network comprising:
a central equipment having an input and an output;
a bi-directional signal transmission line having one end connected
to said input and output of said central equipment; and
a plurality of local equipments connected to said signal
transmission line downstream of said control equipment, and each
having at least one terminal connected to said bi-directional
signal transmission line;
said central equipment comprising means for transmitting, on a time
division basis, data signals addressed to said local equipments to
said signal transmission line, and said local equipments each
comprising means for transmitting a data signal from said terminal
to said central equipment via said signal transmission line in
response to the reception of a data signal transmitted from said
central equipment and addressed to the respective local
equipment;
said central equipment including means for generating control
signals addressed to said local equipments in response to the
reception of signals transmitted from said local equipments, the
control signals being transmitted over said signal transmission
line on a time division basis; and said local equipments each
including means for controlling a transmission start timing of a
data signal to be transmitted to said central equipment via said
signal transmission line in response to a control signal addressed
to the respective local equipment.
2. A bus-shaped local area network comprising:
a central equipment having an input and an output;
a bi-directional signal transmission line having one end connected
to said input and output of said central equipment; and
a plurality of local equipments connected to said signal
transmission line downstream of said control equipment, and each
having at least one terminal connected to said bi-directional
transmission signal line;
said central equipment comprising means for the transmitting, on a
time division basis, data signals addressed to said local
equipments to said signal transmission line, and said local
equipments each comprising means for transmitting a data signal to
said central equipment via said signal transmission line in
response to the reception of a data signal transmitted from said
central equipment and addressed to the respective local
equipment;
said central equipment including:
means for generating test signals addressed to said respective
local equipments, the test signals being transmitted over said
signal transmission line on a time division basis;
means for measuring a delay time between a point of time when a
test signal addressed to each local equipment is transmitted from
said central equipment and a point of time when a signal
transmitted from the local equipment in response to the reception
of the test signal addressed thereto is received by said central
equipment; and
means for generating a control signal for controlling a
transmission start timing of a data signal from each local
equipment according to the delay time measured for the local
equipment, the control signals addressed to said local equipments
being transmitted over said signal transmission line on a time
division basis; and
each local equipment including:
means for detecting a control signal addressed thereto; and
means for controlling the transmission start timing of a data
signal to be transmitted over said signal transmission line in
response to the detected control signal.
3. The local area network according to claim 2, wherein:
the test signals each include a sync signal component having
address information for identifying a local equipment, and a test
control signal component;
said control signal detecting means of each local equipment
includes:
means for detecting a sync signal component addressed to the
respective local equipment;
means for detecting the test control signal component in response
to the detection of the sync signal component addressed thereto by
said sync signal detecting means; and
means responsive to said test control signal detecting means for
transmitting a sync signal identical with the self-addressed sync
signal component over said signal transmission line upon detection
of the test control signal by said test control signal detecting
means;
said delay time measuring means of said central equipment
comprising means for measuring a time interval between a point of
time of transmission of a test signal to each local equipment and a
point of time of reception of the sync signal transmitted from the
local equipment in response to the reception of the test signal;
and
said control signal generating means comprising means for
generating a control signal including a sync signal component and a
timing control data component corresponding to the measured delay
time.
4. The local area network according to claim 3, wherein:
the timing-control data component addressed to each local equipment
has a time control amount such that the time interval between a
point of time of transmission of a data signal from said central
equipment to the local equipment and a point of time of reception
by said central equipment of a data signal transmittted from the
local equipment becomes substantially equal to the time intervals
with respect to other local equipments.
5. The local area network according to claim 2, wherein:
the data signal transmitted from said central equipment to each
local equipment includes time-division multiplexed data components
addressed to said terminals connected to the local equipment;
and
the data signal transmitted from each local equipment to said
central equipment includes time-division multiplexed data
components from said terminals connected to the local
equipment.
6. The local area network according to claim 5, wherein:
each local equipment includes:
a distributor connected to receive the self-addressed time-division
multiplexed data components for distributing the data components to
said terminals; and
a multiplexer for time-division multiplexing data components form
said terminals.
7. The local area network according to claim 5, wherein:
the data signal transmitted from said central equipment to each
local equipment includes a sync signal component containing address
information for the local equipment; and
the data signal transmitted from each local equipment to said
central equipment includes a sync signal component containing
address information for the local equipment.
8. A bus-shaped local area network comprising:
a central equipment having an input and an output;
a bi-directional signal transmission line having one end connected
to said input and output of said central equipment; and
a plurality of local equipments connected to said signal
transmission line downstream of said control equipment, and each
having at least one terminal connected to said signal transmission
line;
said central equipment comprising means for transmitting, on a time
division basis, data signals addressed to said local equipments to
said signal transmission line, said local equipments each
comprising means for transmitting a data signal to said central
equipment via said signal transmission line in response to the
reception of a data signal transmitted from said central equipment
and addressed to the respective local equipment;
said central equipment including:
means for detecting a collision between words when receiving data
signals from said local equipments; and
means responsive to said collision detecting means for generating a
control signal addressed to each local equipment for controlling a
transmission start timing of a data signal to be transmitted from
the local equipment to said central equipment, control signals
addressed to said local equipments being transmitted over said
signal transmission line on a time division basis; and
each local equipment includes:
means for detecting a control signal addressed thereto; and
means for controlling the transmission start timing of a data
signal to said central equipment in response to the detected
control signal.
9. The local area network according to claim 8, wherein:
said control signal generating means of said central equipment
comprises means for generating a control signal each time a
collision between data signals transmitted from local equipments is
detected by said collision detecting means; and
said transmission start timing control means of each local
equipment comprises means for varying the transmission start-timing
of a data signal each time a control signal addressed thereto is
detected.
10. The local area network according to claim 8, wherein:
said control signal generating means comprises means for generating
a control signal instructing the retarding of the transmission
start timing for the local equipment which generated the succeeding
one of two data signals between which a collision occurred.
11. The local area network according to claim 8, wherein:
said central equipment includes overspace detecting means for
detecting an overspace in excess of a predetermined space between
words; and
said control signal generating means comprises means for generating
a control signal instructing the advancement of transmission start
timing for the local equipment which generated the succeeding one
of two data signals between which an overspace is detected by said
overspace detecting means.
12. The local area network according to claim 8, wherein:
the data signal transmitted from said central equipment to each
local equipment includes a sync signal component containing address
information identifying the local equipment, a control signal
component generated by said control signal generating means, and
time-division multiplexed data components addressed to said
terminals connected to the local equipment; and
the data signal transmitted from each local equipment to said
central equipment includes a sync signal component and
time-division multiplexed data components from said terminals
connected to the local equipment.
13. The local area network according to claim 12, wherein:
each local equipment includes:
a distributor connected to receive time-division multiplexed data
components addressed thereto for distributing the data components
to said terminals connected to the local equipment; and
a multiplexer for time-division multiplexing data components from
said terminals.
14. The local area network according to claim 8, wherein:
said transmission start timing control means of each local
equipment includes delay circuit means connected to receive a data
signal to be transmitted over said signal transmission path, the
delay time provided by said delay circuit means being controlled by
the control signal.
15. A bus-shaped local area network comprising:
a central equipment having an input and an output;
a bi-directional signal transmission line having one end connected
to said input and output of said central equipment; and
a plurality of local equipments connected to said signal
transmission line downstream of said central equipment, and each
having at least one terminal connected to said signal transmission
line;
said central equipment comprising means for transmitting, on a time
division basis, data signals addressed to said local equipments to
said signal transmission line, and said local equipments each being
arranged to transmit a data signal to said central equipment via
said signal transmission line in response to the reception of a
data signal transmitted from said central equipment and addressed
to the respective local equipment;
said central equipment including:
means for detecting an overspace in excess of a predetermined space
between data signals when receiving data signals from said local
equipments; and
means responsive to said overspace detecting means for generating a
control signal addressed to each local equipment for controlling a
transmission start timing of a data signal from the local equipment
to said central equipment, the control signals addressed to said
local equipments being transmitted over said signal transmission
path on a time division basis; and
each local equipment including:
means for detecting a control signal addressed thereto; and
means for controlling the transmission start timing of a data
signal to said central equipment in response to the detected
control signal.
16. The local area network according to claim 15, wherein:
said control signal generating means of said central equipment
comprises means for generating a control signal every time an
overspace between data signals from said local equipments is
detected by said overspace detecting means; and
said transmission start timing control means of each local
equipment comprises means for varying the transmission start timing
every time a control signal addressed thereto is detected.
17. The local area network according to claim 16, wherein:
said control signal generating means comprises means for generating
a control signal instructing the advancement of the transmission
start timing of the local equipment which generated the succeeding
one of two data signals between which an overspace is detected.
Description
BACKGROUND OF THE INVENTION
This invention relates to a local area network, and, more
particularly, to a bus-configured local area network with data
exchange capability.
With recent office automation, various electronic devices such as
facsimiles and computers are extensively used, and there is a
demand for a local area network with data exchange capability for
terminals including electronic devices and telephone sets. However,
it is very difficult to directly connect all terminals to a private
branch exchange (PBX), as in an existing network using a PBX.
The difficulty can be alleviated by providing each area, such as a
building or a building floor, with a local equipment to which
terminals are connected. A simplified local-area network
architecture may be realized by connecting each local equipment to
a point of the bus wired from PBX, serving as a central equipment,
instead of directly connecting each local equipment thereto. In
such a bus-configured local-area network, signal transmission and
reception is achieved by a time-division multiplexing system.
The bus-configured local-area network suffers from such a
disadvantage as described below. Since the signal transmission time
between the central equipment and each local equipment varies with
the point at which the local equipment is connected to the bus, and
the signal transmission and reception is done in a time-division
manner, collision and overspace are liable to occur between signals
sent from different local equipments to the central equipment. It
is evident that the collision of signals is undesirable for signal
transmission and reception. The overspace reduces signal
transmission efficiency.
SUMMARY OF THE INVENTION
An object of the invention is to provide an improved bus-configured
local area network.
Another object of the invention is to provide a bus-configured
local area network in which a plurality of local equipments, to
each of which one or more terminals are connected, are connected to
respective points of a bus leading from a central equipment, and
which is arranged to avoid collisions between signals sent out from
the local equipments on the bus.
Still another object of the invention is to provide a
bus-configured local area network in which a plurality of local
equipments, to each of which one or more terminals are connected,
are connected to respective points of a bus leading from a central
equipment, and which is arranged to raise the signal transmission
efficiency.
The local area network, to which the invention is directed,
comprises a central equipment having an input and an output, a
common signal transmission path one end of which is connected to
the input and output of the central equipment, and a plurality of
local equipments each having an input and output connected to a
point on the common signal transmission path, to each of the local
equipments one or more terminals are connected.
The central equipment is arranged to transmit signals addressed to
individual local equipments on the common signal transmission path
in a time division manner. Each of the local equipments is
responsive to reception of the signal addressed thereto to transmit
a data signal to the central equipment via the signal transmission
path.
To attain the objects of the invention, the central equipment
includes means for generating control signals addressed to
respective local equipments in response to the reception of signals
transmitted therefrom, the control signals being transmitted over
the signal transmission path in a time division manner, and each
local equipment includes means for controlling a transmission
timing of a signal to be transmitted to the central equipment in
response to the control signal addressed thereto.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a bus-configured local-area
network according to the invention;
FIG. 2 is a block diagram of a central equipment according to a
first embodiment of the invention;
FIG. 3 is a block diagram of a local equipment according to the
first embodiment of the invention;
FIGS. 4A and 4B show test signals transmitted from the central
equipment to local equipments, and respond signals transmitted from
local equipments to the central equipment in response to the test
signals in a test mode;
FIG. 5 shows transmission-start-timing control signals sent from
the central equipment to local equipments in a transmission
start-timing setting mode;
FIG. 6 is a time chart for explaining the operation of the first
embodiment of the invention;
FIG. 7 is a block diagram showing a test signal and transmission
start timing-control-signal generating circuit of FIG. 2;
FIG. 8 is a block diagram of a distributor, a test signal, a
transmission-start timing-control-signal detecting circuit, and a
transmission-start timing-control circuit FIG. 3;
FIG. 9 is a block diagram of a central equipment according to a
second embodiment of the invention;
FIG. 10 is a block diagram of a local equipment in the second
embodiment;
FIG. 11 shows the format of one word sent from the central
equipment to each local equipment;
FIG. 12 is a block diagram of a collision detecting circuit and an
overspace detecting circuit of FIG. 9;
FIG. 13 is a time chart for explaining the collision detecting
operation;
FIG. 14 is a time chart for explaining the operation of the second
embodiment of the invention;
FIG. 15 is a block diagram of a control-signal generating circuit
of FIG. 9;
FIG. 16 is a block diagram of the multiplexer of FIG. 9;
FIG. 17 is a time chart for explaining the operation of the
multiplexer of FIG. 16; and
FIG. 18 is a transmission timing control circuit of FIG. 10.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, a local area network according to the
invention comprises a central equipment 11, a plurality of local
equipments (concentrator/distributor) 12.sub.1 to 12.sub.4 and at
least one terminal 13 connected to each local equipment. A common
signal transmission path 14, consisting of an up link 14a and a
down link 14b, is cabled from central equipment 11. An output line
15a and an input line 15b of each local equipment are connected to
up and down links 14a and 14b, respectively. The terminals
connected to the local equipments may be telephone sets, facsimiles
or time-sharing system terminals, etc., or a combination of
different kinds of terminals. The common signal transmission path
14 may be an optical signal transmission path, a baseband
transmission path using coaxial cables, or a broadband transmission
path (modulation transmission path) applied CATV techniques.
For the sake of simplicity, four local equipment 12.sub.1 to
12.sub.4 are connected to central equipment 11 in the illustrated
positional relation. The local equipment 12.sub.1 is connected to
path 14 at the nearest point thereof from central equipment 11, and
the local equipment 12.sub.4 is connected to the common bus 14 at
the remotest point from the central equipment 11. The signal
transmission time between central equipment 11 and each local
equipment depends on the position of the local equipment on signal
transmission path 14.
The central equipment 11 is capable of permitting data exchange
between terminals. If the terminals are all telephone sets, it is a
private branch exchange (PBX) in the ordinary sense. Each local
equipment multiplexes signals from the terminals on a time division
basis and sends out the multiplexed signals on up link 14a together
with a synchronizing signal containing address information
identifying the local equipment. The multiplexed signals sent out
from each local equipment constitute one word. The length of one
word depends on the number and type of terminals connected to each
local equipment.
The central equipment 11 receives words transmitted from the local
equipments and performs exchanges of the received data according to
subscriber data. It also transmits, after data exchange, words
addressed to the local equipments on down link 14b in a
predetermined sequence previously allocated to the local equipments
within one frame time. The time length of one frame depends on the
number of local equipments, and the number and type of
terminals.
Each local equipment receives self-addressed word identified by the
work sync signal to distribute the received data to the
corresponding terminals. Each local equipment is arranged to send
out a new word after the reception of the self-addressed word from
the central equipment. For the transmission of data either a
baseband transmission system or a carrier-modulating transmission
system may be used.
In the local area network of FIG. 1 as summarized above, the signal
transmission time between central equipment 11 and a local
equipment varies with the position of the latter. Therefore,
sending a new word on up link 14a from a local equipment, in
response to the reception of a word from the central equipment, may
cause a collision with a word from another local equipment.
According to the invention, the start timing for word transmission
from each local equipment is controlled in such a manner as to
avoid such a collision between words, and also to minimize a space
between words as will be described in detail hereafter.
Referring to FIG. 2 the central equipment according to an
embodiment of the invention is shown, which comprises a receiver
21, a word-sync signal-detecting circuit 22, a distributor 23, an
exchange 24, a multiplexer 25, a transmitter 26, a subscriber's
data memory, and a test signal and transmission start-timing
control-signal generator 28.
The receiver 21 receives a signal transmitted from each local
equipment via up link 14a and decodes the received signal into a
logic data signal. The logic data signal is applied to word-sync
signal-detecting circuit 22, which detects a word sync signal
SI.sub.i (i=1, 2, 3 or 4), indicating the local equipment which
generated the received signal. The logic data signal is also
applied to distributor 23. The distributor 23 is responsive to
word-sync detecting circuit 22 to convert signals serially
transmitted from local equipments 12.sub.1 to 12.sub.4 to parallel
signals corresponding to the respective local equipments. The
parallel signals are applied to exchange 24 in which data exchange
processing is performed in a known manner according to subscriber
data provided by subscriber data memory 27. The subscriber data
includes address information (i.e., word sync signal) SO.sub.i for
identifying the local equipments and data TWi representing the
number of terminals connected to each local equipment (which
determines the length of one word allotted to the corresponding
local equipment 12.sub.i).
The exchange 24 provides signals, addressed to the respective local
equipments 12.sub.1 to 12.sub.4, in parallel, which are applied to
multiplexer 25 together with the subscriber data SO.sub.i. The
multiplexer 25 adds corresponding subscriber data SOi, as a word
sync signal, to data signals addressed to the local equipment
12.sub.i and applies to transmitter 26 words addressed to the
respective local equipments at predetermined timings in a time
division manner. The transmitter 26 transmits these words on down
link 14b in a form suited for transmission. Clock pulses are
superimposed on the transmission signals to establish
synchronization between the central equipment and each local
equipment.
The control signal generating circuit 28 is provided for avoiding
collisions between signals transmitted from local equipments. The
circuit 28 operates in a test mode and a transmission start
timing-setting mode prior to the normal data transmission mode. In
the test mode, it generates test signals addressed to the
respective local equipments to measure, for each local equipment, a
time difference (i.e., delay time) between the instant of
transmission of a test signal from the central equipment 11 and the
instant of reception by central equipment 11 of a responding signal
transmitted from a local equipment in response to the
self-addressed test signal. In the transmission-start
timing-setting mode, the circuit 28 calculates, for each local
equipment, control data for setting the transmission start timing
on the basis of the measured delay time and provides the control
data addressed to the respective local equipments.
In this embodiment, the test signal, transmitted from the central
equipment to each local equipment in the test mode, consists of a
word sync signal SO.sub.i and a common test control signal
DT.sub.t. On the other hand, the responding signal, transmitted
from each local equipment to the central equipment in response to
the self-addressed test signal, consists of a word sync signal
SI.sub.i. The signal, transmitted from the central equipment to
each local equipment in the transmission start timing-setting mode,
consists of word sync signal SO.sub.i and control data DT.sub.i. In
both the test mode and transmission-start timing-setting mode, an
output signal of circuit 28 is applied to transmitter 26 through an
OR gate 29.
Referring to FIG. 3 an example of the local equipment is shown
which comprises a receiver 31, a word sync signal detecting circuit
32, a distributor 33, interfaces 34a to 34c, a multiplexer 35, a
transmitter 36, a test signal and transmission-start timing signal
detecting circuit 37, and a transmission-start timing-control
circuit 38.
The receiver 31 receives a signal transmitted from central
equipment 11 via down link 14b to decode it into a logic data
signal. Simultaneously, clock signals .phi..sub.1 and .phi..sub.2
are recovered. The word sync signal-detecting circuit 32 detects
the word sync signal SO.sub.i from the logic data signal. When the
word sync signal SO.sub.i is detected, the distributor 33 receives
the succeeding data signal DO.sub.i to separate signals addressed
to the respective terminals 13a to 13c, which are time-division
multiplexed in a predetermined order. The separated data signals
are distributed to the corresponding terminals 13a to 13c through
interfaces 34a to 34c. Where each terminal is a telephone set, each
interface may include a digital-to-analog converter for converting
a digital signal supplied from distributor 33 into an analog
signal, an analog-to-digital converter for converting an analog
voice signal into a digital signal, and a hybrid transformer.
The multiplexer 35 time-division multiplexes digital signals from
interfaces 34a to 34c in a predetermined order, and adds the word
sync signal SI.sub.i to the multiplexed signals. An output signal
of multiplexer 35 is transmitted on up link 14a through transmitter
36. In this embodiment, time slots allocated to respective signals
from terminals are equal in length.
As described above, according to the invention, the signal
transmission start timing in each local equipment, i.e., the start
timing in operation of multiplexer 35, is controlled. For this
purpose, the control signal detecting circuit 37 and
transmission-start timing-control circuit 38 are provided. In the
test mode, in response to the detection of a word sync signal by
the word-sync signal-detecting circuit 32, the control signal
detecting circuit 37 detects the test control signal succeeding to
the word sync signal to immediately supply the word sync signal
SI.sub.i through an OR gate 39 to transmitter 36. On the basis of
this signal SI.sub.i, the central equipment 11 measures the delay
time for the corresponding local equipment 12.sub.i. After the
measurement of all the delay times for the local equipments has
been completed, the central equipment calculates the control data
for each local equipment. In the transmission-start timing-setting
mode, the control signal detecting circuit 37 detects the control
data in response to the detection of the word sync signal. The
detected control data is applied to transmission-start
timing-control circuit 38. The control circuit 38 renders the
multiplexer 35 operative at a time when the time length,
represented by the control data, elapses from the time of detection
of the word sync signal by word-sync signal-detecting circuit
32.
The above operation will now be described in more detail. It is now
assumed that the signal transmission times from central equipment
11 to the respective local equipments 12.sub.1 to 12.sub.4 (i.e.,
delay times) are respectively 0.4.DELTA., 0.9.DELTA., 1.1.DELTA.
and 2.3.DELTA.. .DELTA.stands for a proper clock time (for example,
the time length of eight clock pulses). The time lengths TW'.sub.1
to TW'.sub.4 of data signals of the words allocated to local
equipments 12.sub.1 to 12.sub.4 are set according to the subscriber
data TW.sub.i. When a transmission sequence of words WO.sub.1 to
WO.sub.4 from the central equipment 12.sub.1 to 12.sub.4 within one
frame is set, the timing of each word transmission is determined.
The test signals are transmitted to the corresponding local
equipments at the same timings as for corresponding words within
one frame.
As shown in FIG. 4A, for example, the transmission timings of test
signals addressed to local equipments 12.sub.2, 12.sub.3, 12.sub.1,
12.sub.4 are set to t.sub.1, t.sub.2, t.sub.3, t.sub.4,
respectively, within one frame. The test signal consists of word
sync signal SO.sub.i and test control signal DT.sub.t, as described
before. The length of time of the test control signal is set to be
considerably shorter than the length of time of the data signal of
one word. Therefore, after a test signal is transmitted, the
central equipment receives a responding signal to the transmitted
test signal before the next test signal is transmitted. The test
control signal DT.sub.t is formed of a bit stream of, for instance,
"01111111".
In view of the time delay TD.sub.i, i.e., time required for
transmission of a signal from the central equipment to each local
equipment, central equipment 11 receives, for instance, the word
sync signal SI.sub.2 as a responding signal to the test signal,
transmitted at the time t.sub.1 to local equipment 12.sub.2, after
the time delay (0.9.DELTA..times.2+Ts) as shown in FIG. 4B; where
Ts is the processing time in each local equipment from the
reception of the test signal to the transmission of the responding
signal. Likewise, it receives the responding signals SI.sub.3,
SI.sub.1 and SI.sub.4 from the local equipments 12.sub.3, 12.sub.1
and 12.sub.4 after the time delays (1.1.DELTA..times.2+Ts),
(0.4.DELTA..times.2+Ts) and (2.3.DELTA..times.2+Ts), respectively.
The central equipment measures the time delay (2TD.sub.i +Ts) for
each local equipment and detects the maximum time delay
(2TDmax+Ts). In this example, the maximum time delay is
(4.6.DELTA.+Ts) for the local equipment 12.sub.4. In this
embodiment, the transmission start times of local equipments
12.sub. 1 to 12.sub.4 are controlled such that the time delays for
the local equipments become all (2TDmax+Ts). To this end, the
central equipment 11 calculates the difference DT.sub.i between the
maximum time delay (2TDmax+Ts) and the time delay (2TD.sub.i +Ts)
peculiar to each local equipment, depending on the position
thereof, and sends DT.sub.i as control data to corresponding local
equipment. The control data DT.sub.i is represented by "0xxxxxxx".
The first bit "0" in the test control signal DT.sub.t and control
data DT.sub.i is provided for distinguishing the succeeding bits
from the normal data of a word. The seven bits represented by "x"
("1" or "0") of the control data, represents an extra time delay to
be added to the peculiar time delay of each local equipment. In
this example, the extra time delays for the local equipments
12.sub.1 to 12.sub.4 are 3.8.DELTA.
(=4.6.DELTA.-0.4.DELTA..times.2), 2.8.DELTA.
(=4.6.DELTA.-0.9.DELTA..times.2), 2.4.DELTA.
(=4.6.DELTA.-1.1.DELTA..times.2) and 0
(=4.6.DELTA.-2.3.DELTA..times.2). The control data DT.sub.i thus
calculated is sent from transmitter 26 together with the
corresponding word sync signal SO.sub.i at the same timing as the
test signal, as shown in FIG. 5. Each local equipment controls the
transmission start time according to the corresponding control data
transmitted from central equipment 11. As a result, the difference
between the transmission time of a word at the central equipment
and the reception time of a corresponding responding word at the
central equipment becomes same with respect to all the local
equipments.
By setting the time delay of each local equipment in the signal
transmission as described above, efficient word transmissions from
the local equipments in the normal data transmission mode can be
performed. This will be described in detail with reference to the
time chart of FIG. 6. The data signal DO.sub.i, DI.sub.i of each
word is formed of "1xxx . . . x".
It is assumed that words WO.sub.2, WO.sub.3, WO.sub.1 and WO.sub.4
are sent to local equipments 12.sub.2, 12.sub.3, 12.sub.1 and
12.sub.4 at times t.sub.1, t.sub.2, t.sub.3 and t.sub.4, as shown
in FIG. 6A. As shown in FIG. 6B, the word WO.sub.2 reaches receiver
31 of local equipment 12.sub.2 after the lapse of a signal
propagation time of 0.9.DELTA., and then, after the lapse of Ts a
responding word WI.sub.2 is produced by multiplexer 35. The signal
transmission from multiplexer 35 is delayed by a time of
2.8.DELTA.. Therefore, the responding word WI.sub.2 appears on up
link 14a with a time delay of (0.9.DELTA.+2.8.DELTA.+Ts) after
t.sub.1. Likewise, responding words WI.sub.3, WI.sub.1 and WI.sub.4
appear on up link 14a with time delays of
(1.1.DELTA.+2.4.DELTA.+Ts), (0.4.DELTA.+3.8.DELTA.+Ts) and
(2.3.DELTA.+Ts), respectively, as shown in FIGS. 6C, 6D and 6E.
These responding words WI.sub.2, WI.sub.3, WI.sub.1, WI.sub.4 on up
link 14a arrive at receiver 21 in central equipment 11 with time
delays of 0.9.DELTA., 1.1.DELTA., 0.4.DELTA. and 2.8.DELTA.,
respectively. That is, times of (0.9.DELTA..times.2+2.8.DELTA.+Ts),
(1.1.DELTA..times.2+2.4.DELTA.+Ts),
(0.4.DELTA..times.2+3.8.DELTA.+Ts) and (2.3.DELTA..times.2+Ts) are
required from the transmissions of the respective words WO.sub.2,
WO.sub.3, WO.sub.1 and WO.sub.4 at central equipment 11 till the
receptions of the responding words WI.sub.2, WI.sub.3, WI.sub.1 and
WI.sub.4 at central equipment 11. In other words, the responding
words WI.sub.2, WI.sub.3, WI.sub.1 and WI.sub.4 arrive at central
equipment 11 with the time delay of (4.6.DELTA.+Ts) after the
transmissions of the corresponding words WO.sub.2, WO.sub.3,
WO.sub.1 and WO.sub.4, as shown in FIG. 6F. It will be understood,
therefore, that words can be reliably and efficiently transmitted
from local equipments 12.sub.1 to 12.sub.4 to central equipment 11
without the possibility of collisions or overspaces between
words.
In order to reliably avoid signal collisions due to possible
variations of time delays in the signal transmission path and local
equipment, a time space of about 0.1.DELTA. may be provided between
words transmitted from the local equipments. In this embodiment, a
space Sp is provided at the end of each frame to facilitate
distinguishing between frames. The space Sp, however, may be
omitted, because frames can be distinguished by the previously
known word sync signal of the first word in the frame. In this
embodiment, the maximum time delay (2TDmax+Ts) among the delay
times associated with the local equipments is found, by the maximum
time delay may be previously determined to be longer than
(2TDmax+Ts).
Now, an embodiment of control-signal generating circuit 28 will be
described with reference to FIG. 7. The circuit 28 includes control
signal sending means 28.sub.1, delay time measuring means 28.sub.2
and test signal/transmission-start time control-signal generating
means 28.sub.3. The clock .phi..sub.2 has, for example, about 10
times the frequency of clock .phi..sub.1. The control-signal
sending means 28.sub.1 has a shift register 41. In the test mode,
subscriber data (word sync signal) SO.sub.i from subscriber data
memory 27 and test control signal DT.sub.i from generating means
28.sub.3 are loaded into shift register 41 at the transmission time
of the word WO.sub.i to local equipment 12.sub.i. The test signal
(SO.sub.i +DT.sub.t), loaded in shift register 41, is read out bit
by bit by clock pulses .phi..sub.1 supplied through an AND gate 42,
which is enabled by a monostable multivibrator 43 whose output goes
high for a period of time equal to the test signal. A counter 44
and a comparator 45 are provided for measuring the length of time
of word WO.sub.i and triggering monostable multivibrator 43. The
counter 44 counts clock pulses .phi..sub.1 whose count value is
compared, by comparator 45, with the subscriber data TW.sub.i from
subscriber data memory 27. The subscriber data TW.sub.i represents
the length of time of word WO.sub.i, which depends on the number
and type of terminals 13 connected to local equipment 12.sub.i.
When the count of counter 44 coincides with subscriber data
TW.sub.i, the comparator 45 triggers monostable multivibrator 43
and clears counter 44. The monostable multivibrator 43 is arranged
to, when triggered, provide a high level output during the period
of time necessary for transmission of the test signal.
The time delay measuring means 28.sub.2 includes a counter 46 and a
latch 47. The counter 46 counts clock pulses .phi..sub.2 and is
cleared by comparator 45. The count of counter 46 is latched in
latch 47 in response to a detection signal (SI.sub.i) of word-sync
signal-detecting circuits 2 for word sync signal SI.sub.i from
local equipment 12.sub.i. Since counter 46 is cleared at the same
time that monostable multivibrator 43 is triggered by comparator
45, the count of counter 46, latched in latch 47, represents the
time interval from the transmission of the test signal (SO.sub.i
+DT.sub.t) until the reception of the word sync signal SI.sub.i,
i.e., the time delay (2TD.sub.i +Ts) associated with local
equipment 12.sub.i.
The content of latch 47 representing the measured delay time is fed
to a microcomputer 48, which constitutes control signal generating
means 28.sub.3. The microcomputer 48 detects the maximum time delay
(2TDmax+Ts) among the time delays measured for local equipments
12.sub.1 to 12.sub.4 and then calculates the control data DT.sub.i
for each local equipment 12.sub.i, as stated above.
In the transmission-start time-setting mode, the calculated control
data DT.sub.i is loaded into shift register 41 together with the
word sync signal SO.sub.i, as in the case of the test control
signal DT.sub.t, and then sent out therefrom.
At the beginning of the test mode and transmission-start
time-setting mode, shift register 41, monostable multivibrator 43,
counters 44 and 46, and latch 47 are initialized by microcomputer
48. When initialized, monostable multivibrator 43 provides a high
level output for a predetermined period of time. The subscriber
data SO.sub.i, TW.sub.i, test control signal DT.sub.t, and control
data DT.sub.i are supplied to control signal sending means 28.sub.1
at predetermined times (t.sub.1, t.sub.2, t.sub.3, t.sub.4) under
the control of microcomputer 48.
Referring to FIG. 8, the distributor 33, control signal detecting
circuit 37 and transmission-start timing-control circuit 38 will be
described hereinbelow.
In each local equipment, it is necessary to discriminate between
normal data "1xxx . . . x" and control signal "01111111" or
"0xxxxxxx". The distributor 33 is arranged to receive only data
"xxx . . . x" by making use of a bit "1" following the word sync
signal. It includes a type-D flip-flop 51, which receives received
data at its data input and is clocked by a word-sync
signal-detection signal (SO.sub.i). When a word sync signal
SO.sub.i is detected in the normal data transmission mode, the
flip-flop 51 provides at its output the bit "1", following the word
sync signal, to trigger monostable multivibrator 52.
When triggered, the multivibrator 52 is arranged to provide a
high-level output to enable an AND gate 53 for a length of time of
the word data part DO.sub.i. Thus, when a word sync signal SO.sub.i
is detected in the transmission mode, the following data "xxx . . .
x" is supplied through AND gate 53 to distributing circuit 54. In
the case of the control signal, the bit following the word sync
signal SO.sub.i is "0". Thus, in this case, the monostable
multivibrator 52 is not triggered and the control signal is not
supplied to distributing circuit 54.
The control signal detecting circuit 37 includes a monostable
multivibrator 55 and a shift register 56, which respectively
receive the sync signal detection signal (SO.sub.i) and received
data from receiver 31. The monostable multivibrator 55 is triggered
by the detection signal (SO.sub.i) and, as a result, its output
goes high for the length of time of the control signal DT.sub.t or
DT.sub.i. The shift register 56 is clocked by clock pulses
.phi..sub.1 to read in the received data serially and read out in
parallel. The output of shift register 56 is coupled to comparators
57 and also to a latch 58. The comparators 57 compares an output
signal of shift register 56 with the test control signal "01111111"
and control data "0xxxxxx" to provicde coincidence signals
(DT.sub.t) and (DT.sub.i). The coincidence signal (DT.sub.t), which
is obtained when the output signal of shift register 36 coincides
with the test control signal DT.sub.t, triggers a monostable
multivibrator 60 through an AND gate 59, which is enabled by
monostable multivibrator 55. Thus, the output of multivibrator 60
goes high to enable an AND gate 61 for the length of time of the
word sync signal SI.sub.i. During this time, clock pulses
.phi..sub.1 are supplied through AND gate 61 to a shift register
62. The shift register 62 receives the bits of the word sync signal
SI.sub.i in parallel and sends out them serially in response to
clock pulses .phi..sub.1. Thus, it is understood that, in the test
mode, when each local equipment 12.sub.i receives the test signal
SO.sub.i +DT.sub.t, it sends out the word sync signal SI.sub.i as a
responding signal to the central equipment 11.
When the output signal of shift register 56 coincides with the
control data DT.sub.i "0xxxxxxx" in the transmission-start
timing-setting mode, the comparators 57 provides the coincidence
signal (DT.sub.i), which is fed through an AND gate 63 to latch 58,
whereby the output signal shift register 56 is latched in latch 58.
The detection of the control data DT.sub.i in comparators 57 can be
performed on the basis of a fact that the first bit of control data
DT.sub.i is "0", and the remaining bits always include at least one
bit "0".
The control data DT.sub.i latched in latch 58 is fed to a
comparator 64 in transmission-start timing-control circuit 38. The
circuit 38 also includes a counter 65, which counts clock pulses
.phi..sub.2 and is cleared by the word-sync signal-detection signal
(SI.sub.i). The comparator 64 compares the count of counter 65 with
control data DT.sub.i. When a coincidence occurs, the comparator 64
produces an output signal to actuate multiplexer 35. As a result,
the transmission of signal is started. It will be understood,
therefore, that, in the normal data transmission mode, the signal
transmission is started after the time represented by the control
data DT.sub.i latched in latch 58 in the transmission-start
timing-setting mode has been passed from a point of time at which
the word sync signal SO.sub.i is detected.
Now, another embodiment of the invention will be described. In this
embodiment, the central equipment checks whether there is any
collision or overspace between words transmitted from local
equipments and adds corresponding control signals to words to be
sent to the local equipments. As a result, in each local equipment,
the signal transmission timing is controlled such that neither
collision nor overspace occurs between words from local
equipments.
FIG. 9 shows an arrangement of the central equipment in this
embodiment. Like parts, as in the central equipment in the
preceding embodiment shown in FIG. 2, are designated by the same
reference numerals with a suffix a.
Signals transmitted over up link 14a by local equipment 12.sub.1 to
12.sub.4 are applied to a receiver 21a, a collision detecting
circuit 71, and an overspace detecting circuit 72. The collision
detecting circuit 71 detects collisions between words from local
equipments 12.sub.1 to 12.sub.4 according to a change in the level
of a signal transmitted over up link 14a, which results from a
collision as will be described later. The circuit 71 is arranged
to, when detecting a collision, produce an output of "1". The
overspace detecting circuit 72 detects an overspace between words
from local equipments, and produces an output of "1" upon detection
of an overspace. The collision-detection signal CD of circuit 71
and overspace-detection signal OSD of circuit 72 are applied to a
control-signal generating circuit 73, together with a word sync
signal detecting signal (SI.sub.i) of word-sync signal-detecting
circuit 22a. The control signal generating circuit 73 produces a
control signal CT.sub.i which is applied to a multiplexer 25a. The
control signal CT.sub.i is a 2-bit signal consisting of the
collision detection signal CD and overspace detection signal OSD.
The multiplexer 25a time-division multiplexes words addressed to
local equipments. The words transmitted to local equipments each
have a format as shown in FIG. 11, consisting of a word sync signal
SO.sub.i stored in multiplexer 25a, a control signal CT.sub.i from
control-signal generating circuit 73, and data signal DO.sub.i from
exchange 24a. The words, transmitted from the local equipments to
the central equipment, each consist of a word sync signal SI.sub.i
and data signal DI.sub. i, as in the previous embodiment. The
time-division multiplexed words for the local equipments constitute
one frame. The word sequence in the frame is predetermined.
Therefore, it is possible for each local equipment to receive a
self-addressed word by arranging a frame sync signal FSY at the
head of the frame instead of adding a word sync signal to each
word.
FIG. 10 shows a structure of the local equipment. A control signal
detecting circuit 74 is provided which is responsive to the
detection of the word sync signal SO.sub.i by sync signal detecting
circuit 32a to detect the control signal CT.sub.i succeeding to the
sync signal. The detected control signal CT.sub.i is supplied to a
transmission start-timing control circuit 75, which is connected
between multiplexer 35a and transmitter 36a. In response to the
detection of the word sync signal SO.sub.i by sync signal detecting
circuit 32a, the multiplexer 35a starts the time-division
multiplexing of signals from terminals 13a to 13c to supply the
signal to transmission start-time control circuit 75.
The transmission start-time control circuit 75 controls the
start-time of signal supply from mutliplexer 35a to transmitter 36a
in response to the detected control signal CT.sub.i. In other
words, the control circuit 75 controls a delay time T.sub.i from a
point of time when the word sync signal is detected to a point of
time when the word is actually transmitted from transmitter 36a
over up link 14a. In transmission-start timing-control circuit 75
of each local equipment, a common initial value of the delay time
T.sub.i is set. When the control signal CT.sub.i from control
signal detecting circuit 74 is indicative of a collision, the
control circuit 75 increases the delay time T.sub.i retard the
transmission of a corresponding word. On the other hand, when the
control signal CT.sub.i is indicative of an overspace, it reduces
the delay time T.sub.i to advance the transmission of the
corresponding word. When the control signal CT.sub.i is indicative
of neither collision nor overspace, the delay time T.sub.i is held
at the initial value.
When a collision between two words on the up link 14a is detected
in the central equipment, the word sync signal of the preceeding
word can be detected, while the word sync signal of the succeeding
word cannot be detected. However, since the sequence of words in
one frame can be known, the control signal CT.sub.i indicative of a
collision occurrence can be added to a word addressed to the local
equipment, which transmitted the succeeding word in collision.
Thus, the delay time T.sub.i of the said local equipment
increases.
The signal transmitted over signal transmission path 14 may be
encoded and modulated by various systems, and the methods of
collision detection varies with the adopted encoding and modulating
systems. A collision detection method in case where a bi-phase
encoding system is adopted will now be described with reference to
FIGS. 12 and 13.
FIG. 12 shows the collision detecting circuit 71 and overspace
detecting circuit 72. The collision detecting circuit 71 includes a
lowpass filter (LPF) 81 connected to receive a signal transmitted
over up link 14a, a comparator 82 for comparing an output voltage
of lowpass filter 81 with a first threshold voltage SH.sub.1, and a
monostable multivibrator 83 connected to the output of comparator
82. When the lowpass-filter output voltage exceeds the threshold
level SH.sub.1, the comparator 82 produces an output of "1" to
trigger monostable multivibrator 83, which thus produces a
collision detection signal CD of a logic level "1" indicative of a
collision.
In overspace detecting circuit 72, a comparator 84 compares the
lowpass-filter output voltage with a second threshold voltage
SH.sub.2 to produce an output voltage of a logic level "1" while
the lowpass-filter output voltage is lower than the voltage
SH.sub.2. The comparator output is fed to a timer 85. The timer 85
measures the duration of the "1" level output voltage of comparator
84 to generate an output voltage corresponding to the duration. For
example, this may be performed by counting clock pulses over the
duration of the "1" level output voltage of comparator 84 and
converting the count value into an analog voltage. The output
voltage of timer 85 corresponds to a space between words, as will
be described later in detail. A comparator 86 compares this voltage
with a threshold voltage Sm corresponding to the maximum
permissible space. When the output voltage of timer 85 exceeds the
voltage Sm, the comparator 86 generates an output voltage of a
logic level "1" to trigger a monostable multivibrator 87, which
thus generates an overspace detection signal OSD of a logic level
"1".
Assume now that a logic data signal "0101110010" is transmitted.
The bits of the data signal are generated at respective times
t.sub.1 to t.sub.11, as shown in FIG. 13A. This data signal is
encoded by bi-phase encoding system, whereby an encoded signal, as
shown in FIG. 13B, is sent out. When any signal is not transmitted
over the transmission path the signal level on the path is zero.
When a signal is transmitted, on the other hand, a DC level
prevails on the path. Thus, when a signal, as shown in FIG. 13B is
fed to lowpass filter 81 of FIG. 12, a DC output, as shown in FIG.
13C, appears at its output.
Assume now that two signals, as shown in FIGS. 13D and 13E, are
simultaneously sent out over up link 14a. These signals are
superimposed on each other, and the resultant composite signal, as
shown in FIG. 13F, enters lowpass filter 81. Consequently, the
lowpass filter 81 provides an output voltage, as shown in FIG. 13G,
with the peak value thereof reaching as high as substantially
double the level in the absence of a collision (FIG. 13C).
The threshold voltage level SH.sub.1 of comparator 82 of FIG. 12 is
set to substantially 1.5 times the DC output voltage of lowpass
filter 81 that is obtained in the absence of collision. When a
collision occurs, the output of comparator 82 thus goes to a logic
level "1", causing monostable multivibrator 83 to produce a
collision-detection signal CD with a constant duration.
The threshold voltage level SH.sub.2 of overspace detecting circuit
84 is set to substantially 0.5 D. During the space between words,
the output voltage of lowpass filter 81 is zero volt. When the
output voltage of lowpass filter 81 is below 0.5 D, the output of
comparator 84 goes high, representing the occurrence of a space.
The timer 85 measures the duration of the space. The comparator 86
compares this duration with the maximum permissible duration of
space. When the duration of the space measured by timer 85 exceeds
Sm, a monostable multivibrator 87 produces an overspace detection
signal OSD having a fixed duration. The maximum permissible
duration of space is desirably 1.2.DELTA..
The control operation in this embodiment will now be described with
reference to FIG. 14. In this case, it is assumed that the signal
transmission times between central equipment 11 and local
equipments 12.sub.1 to 12.sub.4 are 1.DELTA., 3.DELTA., 3.5.DELTA.
and 7.DELTA., respectively. Also it is assumed that the sequence of
words WO.sub.1 to WO.sub.4, in one frame, transmitted from central
equipment 11 to local equipments 12.sub.1 to 12.sub.4 is as shown
in FIG. 14A.
In this example, the transmission start time control is performed
with a point of time of detection of a self-addressed word sync
signal SO.sub.i by each local equipment taken as a reference time.
The time interval from the reception of the self-addressed word to
the detection of the word sync signal SO.sub.i is set to T's for
all the local equipments. It is further assumed that each local
equipment 12 starts to transmit a signal over up link 14a at time
T.sub.i (i=1, 2, 3, 4) after the reference time. The time T.sub.i
is a delay time which is controlled by transmission-start
timing-control circuit 75. It is further assumed that the words
WO.sub.1 to WO.sub.4 transmitted from the central equipment 11 to
the local equipments have an equal word length, and also the words
WI.sub.1 to WI.sub.4 transmitted from the local equipments to the
central equipment have an equal word length.
Several algorithms are conceivable for the timing control in
transmission-start timing-control circuit 75. A time control
according to a first algorithm will now be described. The first
algorithm is as follows.
(1) The transmission start timing of the first word WI.sub.3 in the
frame is not controlled.
(2) When a collision between words occurs, the delay time in the
local equipment, which generated the succeeding word in collision,
is increased by .DELTA..
(3) When an overspace occurs between words, the delay time in the
local equipment which generated the succeeding word is reduced by
.DELTA..
The initial value of the controllable delay time T.sub.i of each
local equipment is assumed to be 5.DELTA.. When the words WO.sub.3,
WO.sub.1, WO.sub.4 and WO.sub.2 are sequentially transmitted, as
shown in FIG. 14A, on down link 14b, the local equipments 12.sub.3,
12.sub.1, 12.sub.4 and 12.sub.2 transmit words WI.sub.3, WI.sub.1,
WI.sub.4 and WI.sub.2 on up link 14a at timings as shown in FIGS.
14B to 14E. These timings are each determined by the signal
transmission time from the central equipment to each local
equipment, time Ts required for the word sync signal detection, and
delay time T.sub.i. In FIGS. 14B to 14E, time intervals T.sub.3,
T.sub.1, T.sub.4 and T.sub.2 are equal to 5.DELTA., which is the
initial value of the delay time T.sub.i. The word transmitted from
each local equipment on up link 14a reaches central equipment 11
after the lapse of the corresponding signal-transmission time. FIG.
14F shows the timings at which the words WI.sub.3, WI.sub.1,
WI.sub.4 and WI.sub.2 arrive at central equipment 11. It will be
seen that collisions Q1 and Q2, shown as shadded, and overspaces S1
and S2 occur so long as the delay time T.sub.i is held at the
initial value. The lengths of time (i.e., inter-word times) of the
collisions and overspaces are shown in Table 1 on a row
corresponding to n.sub.c =0 (n.sub.c : times of effective control
operations performed by transmission start-timing control circuit
75).
TABLE 1
__________________________________________________________________________
(The delay time and inter-word time are shown in .DELTA..)
Inter-word Control Delay time time signal FIG. T.sub.3 T.sub.1
T.sub.4 T.sub.2 S.sub.31 S.sub.14 S.sub.42 S.sub.23 CT.sub.3
CT.sub.1 CT.sub.4 CT.sub.2 14
__________________________________________________________________________
Times 0 5 5 5 5 -5 10 -6 6 0 + - + (F) n.sub.c of 1 5 6 4 6 -4 8 -4
5 0 + - + (G) control 2 5 7 3 7 -3 6 -2 4 0 + - + opera- 3 5 8 2 8
-2 4 0 3 0 + - + (H) tion 4 5 9 1 9 -1 2 2 2 0 + - - (I) 5 5 10 0 8
0 0 2 3 0 + + - (I) 6 5 11 1 7 1 0 0 4 0 0 + + 7 5 11 2 8 1 1 0 3 0
0 0 + 8 5 11 2 9 1 1 1 2 0 0 0 0 (K)
__________________________________________________________________________
The inter-word time S.sub.ij represents the time interval between
two consequtive words WI.sub.i and WI.sub.j. It is indicative of a
collision when it is negative, while it is indicative of an
overspace when it is positive. For example, S.sub.31 =-5.DELTA., in
the row for n.sub.c =0 in Table 1, is indicative of a collision Q1
having a length of time of -5.DELTA. between words WI.sub.3 and
WI.sub.1, and S.sub.14 =10.DELTA. is indicative of an overspace S1
having a length of time of 10.DELTA. between words WI.sub.1 and
WI.sub.4.
When the signal shown in FIG. 14F enters central equipment 11, the
collisions Q1 and Q2 are detected by collision detecting circuit
71, while the overspaces S1 and S2 are detected by overspace
detecting circuit 72. The word sync signals SI.sub.3 and SI.sub.4
of words WI.sub.3 and WI.sub.4 are detected by word-sync
signal-detecting circuit 22a. However, the word sync signals
SI.sub.1 and SI.sub.2 of words WI.sub.1 and WI.sub.2 cannot be
detected due to collisions Q1 and Q2. The control-signal generating
circuit 73 decides that a collision occurs between words WI.sub.3
and WI.sub.1 due to word WI.sub.1, and generates control signal
CT.sub.1. The control signal CT.sub.1 is expressed as "+". For the
word WI.sub.2, the control-signal generating circuit 73 also
generates a control signal CT.sub.2 (+) indicative of a
collision.
For the overspace S1 between words WI.sub.1 and WI.sub.4, the
control signal generating circuit 73 generates a control signal
CT.sub.4 for word WI.sub.4. This control signal CT.sub.4 is
expressed as "-", indicating that the overspace detection signal
OSD is "1".
The overspace S2 is detected between the last word WT.sub.2 in a
frame and the first word WI.sub.3 in the next frame. In this case,
the control signal CT.sub.3 is not generated with respect to word
WI.sub.3. This is because, according to the first control
argorithm, the transmission start timing of the first word in the
frame is not controlled. At this time, CT.sub.3 is represented by
"0". The control signal CT.sub.i, generated in the manner as
described, is added to the corresponding word WO.sub.i and then
sent to the corresponding local equipment 12.sub.i. In the local
equipment 12.sub.i, the control-signal detecting circuit 74 detects
the control signal CT.sub.i which has two bits of a collision
detection signal CD and overspace detection signal OSD.
When the collision detection signal CD is "1" (control signal
CT.sub.i is represented as "+"), the transmission start-timing
control circuit 75 increases the delay time T.sub.i by one unit
time .DELTA. to retard the transmission of word WI.sub.i. On the
other hand, when the overspace detection signal OSD is "1" (control
signal CT.sub.i is represented by "-"), it reduces the delay time
T.sub.i by .DELTA. to advance the start of transmission of word
WI.sub.i. When both the detection signals CD and OSD are "0"
(control signal CT.sub.i is represented as "0"), the delay time
T.sub.i remains unchanged.
With the first timing control operation performed according to
control signals when n.sub.c =0, the delay times T.sub.3, T.sub.1,
T.sub.4 and T.sub.2 of local equipments 12.sub.3, 12.sub.1,
12.sub.4 and 12.sub.2 are changed by 0, +.DELTA., -.DELTA. and
+.DELTA., respectively. Consequently, the delay times T.sub.3,
T.sub.1, T.sub.4 and T.sub.2 for n.sub.c =1 become 5.DELTA.,
6.DELTA., 4.DELTA. and 6.DELTA., respectively. As a result of this
control operation, the condition of signal reception in the central
equipment 11 becomes as shown in FIG. 14G. It is is to be noted
that the lengths of time of the collisions Q1 and Q2 and overspaces
S1 and S2 are all reduced. The control operation as described is
repeatedly performed until the 8th control operation (for n.sub.c
=8) is completed. At this time, the inter-word times S.sub.31,
S.sub.14, S.sub.42 and S.sub.23 are .DELTA., .DELTA., .DELTA. and
2.DELTA., respectively.
According to the control argorithm described above, an inter-word
time of zero is regarded as the occurrence of a collision (see FIG.
14J). To this end, the collision detecting circuit 71 may be
modified to produce a collision detection signal of "1" when the
output voltage of lowpass filter 81 is above a predetermined level
over a length of time longer than a predetermined word length.
The minimum value of space Sp between the last word WO.sub.2 in a
frame and the first word WO.sub.3 in the next frame, as shown in
FIG. 14A, is determined by an allowable space between words
transmitted from local equipments 12.sub.1 to 12.sub.4 to central
equipment 11.
The control signal generating circuit 73 will now be described with
reference to FIG. 15. The word number information (i.e. addrerss
information) detected by word sync signal detecting circuit 22a is
temporarily stored in a memory 91. The word number information
temporarily stored in memory 91 is applied to a decoder 92. The
decoder 92 has output terminals 93.sub.1 to 93.sub.4 equal in
number to the words in one frame. The decoder is arranged to decode
the word number information of word WI.sub.i and generate a signal
of "1" at an output corresponding to a word WI.sub.j to be
transmitted next to the word WI.sub.i. In the case of FIG. 14, for
example, when the word number information of word WI.sub.3 is
supplied to decoder 92, the decoder 92 generates a signal of "1" at
the output terminal 93.sub.1 corresponding to the next word
WI.sub.1.
2-bit shift registers 94.sub.1 to 94.sub.4 are provided for the
respective outputs 93.sub.1 to 93.sub.4 of decoder 92. The first
output 93.sub.1 of decoder 92 and the collision detection signal CD
from collision detecting circuit 71 are ANDed by an AND gate
95.sub.1 to set a first stage of shift register 94.sub.1. The first
decoder output 93.sub.1 and the overspace detection signal OSD are
ANDed by an AND gate 96.sub.1 to set a second stage of shift
register 94.sub.1. When the first decoder output 93.sub.1 and
collision detection signal CD are both "1", the first bit Q of
shift register 94.sub.1 is set to "1". Likewise, when the first
decoder output 93.sub.1 and overspace detection signal OSD are both
"1", the second bit D of shift register 94.sub.1 is set to "1". The
input of second stage of shift register 94.sub.1 is grounded, so
that both the first and second bits are "0" unless the outputs of
both the AND gates 95.sub.1 and 96.sub.1 are "1".
Like the AND gates 95.sub.1 and 96.sub.1, AND gates 95.sub.2 to
95.sub.4 and 96.sub.2 to 96.sub.4 are provided for the respective
registers 94.sub.2 to 94.sub.4. The shift registers 94.sub.1 to
94.sub.4 generate control signals CT.sub.1 to CT.sub.4
corresponding to the words WI.sub.1 to WI.sub.4, respectively.
When a collision occurs between words WI.sub.i and WI.sub.j, the
word sync signal SI.sub.j of the succeeding word WI.sub.j cannot be
detected. Therefore, the content of memory 91 remains unchanged,
and a control signal CT.sub.k for a word WI.sub.k next to the word
W.sub.j cannot be produced. Accordingly, the memory 91 must be
rewritten when a collision occurs between two words. To this end, a
rewriting circuit 97 is provided for rewriting the content of
memory 91 in response to the collision detection signal CD. It is
arranged to, when a collision occurs between words WI.sub.i and
WI.sub.j, rewrite the word number information of the word WI.sub.j
stored in memory 91 to the word number information of word WI.sub.k
next to word WI.sub.j. As a result, a correct control signal can be
generated for the word WI.sub.k.
The operation of control signal generating circuit 73 will now be
described with reference to FIG. 14. When collision Q1 occurs
between words WI.sub.3 and WI.sub.1, as shown in FIG. 14F, the
first bit Q of shift register 94.sub.1 is set, so that a control
signal CT.sub.1 of "10" is obtained from shift register 94. This
signal "10" corresponds to a control signal "+" indicative of a
collision, as shown in Table 1.
With this collision Q1, the memory 91 is rewritten to store the
word number information of the word WI.sub.4 next to the word
WI.sub.1. When overspace S1 between words WI.sub.1 and WI.sub.4 is
detected, the second bit of shift register 94.sub.4 is set so that
a control signal CT.sub.4 of "01" is obtained. This signal "01"
corresponds to a signal "-", as shown in Table 1. The word sync
signal of the word WI.sub.4 can be detected, so that the word
number information thereof is stored in memory 91. With collision
Q2 occurring between words WI.sub.4 and WI.sub.2, the first bit of
shift register 94.sub.2 is set so that a control signal CT.sub.2 of
"10" is generated, and the memory 91 is rewritten to store the word
number information of the word WI.sub.3.
For the overspace S2 between the word WI.sub.2 and the first word
WI.sub.3 in the next frame, the shift register 94.sub.3 generates a
control signal CT.sub.3 of "01". This control signal CT.sub.3 is
changed from "01" to "00" by suitable means. This is done because,
according to the argorithm mentioned earlier, the transmission
start timing of the first word in the frame is not controlled.
Alternatively, the shift register 94.sub.3 for the first word may
be arranged so that it cannot be set by suitable means.
The multiplexer 25a will now be described with reference to FIG.
16. Data signals DO.sub.1 to DO.sub.4 from exchange 24a are
temporarily stored in memories 101.sub.1 to 101.sub.4. Word sync
signal generators 102.sub.1 to 102.sub.4 are provided for
generating respective word sync signals SO.sub.1 to SO.sub.4. The
word sync signals SO.sub.1 to SO.sub.4, control signals CT.sub.1 to
CT.sub.4, and data DO.sub.1 to DO.sub.4 are applied to and AND-OR
gate 103. A timing signal generating circuit 104 is provided which
receives clock signal synchronized with exchange 24 and generates
timing signals as shown in FIGS. 17A to 17L, which are also fed to
AND-OR gate 103. The AND-OR gate 103 generates one-frame
transmission signal, as shown in FIG. 17M.
The transmission-start timing-control circuit 75 will now be
described with reference to FIG. 18. It includes an up/down counter
106, shift registers 107.sub.1 to 107.sub.4, AND-OR gates 108.sub.1
to 108.sub.4, and inverters 109.sub.1 to 109.sub.4. The shift
registers 107.sub.1 to 107.sub.4 have one, tow, four and eight
stages, respectively, and thus provide delay times 1.DELTA.,
2.DELTA., 4.DELTA. and 8.DELTA. to the respective input signals
thereof.
The output of AND-OR gate 108.sub.1 is coupled to transmitter 36a.
The outputs of AND-OR gates 108.sub.2 to 108.sub.4 are coupled to
the respective inputs of registers 107.sub.1 to 107.sub.3. The
output of multiplexer 35a is coupled to the input of shift register
107.sub.4. The input and output of shift register 107.sub.1 are
coupled to two inputs of AND-OR gate 108.sub.1. Likewise, the input
and output of shift registers 107.sub.2 to 107.sub.4 are coupled to
two inputs of respective AND-OR gates 108.sub.2 to 108.sub.4.
The collision detection signal CD is applied to an up input of
up/down counter 106, while the overspace detection signal OSD is
applied to its down input. The up/down counter 106 provides four
outputs Q.sub.1, Q.sub.2, Q.sub.4 and Q.sub.8 with respective
weights of 1, 2, 4 and 8. The initial count of up/down counter 106
is set to 5, which corresponds to the initial value 5.DELTA. of the
delay time T.sub.i described above. The outputs Q.sub.1, Q.sub.2,
Q.sub.4 and Q.sub.8 of up/down counter 106 are coupled to AND-OR
gates 108.sub.1 to 108.sub.4 as shown.
With the control circuit 75 arranged as above, when the count of
up/down counter 106 is 5, the input signal from multiplexer 35a is
applied to transmitter 36a through shift registers 107.sub.3 and
107.sub.1. That is, a delay time of 5.DELTA. is provided for the
input signal. Every time a collision detection signal CD is
supplied, the up/down counter 106 increments, so that the delay
time T.sub.i provided for an input signal increases by 1.DELTA.. On
the other hand, every time an overspace detection signal OSD is
supplied, the up/down counter 106 decrements, thus reducing the
delay time T.sub.i by 1.DELTA..
Some other timing-control argorithms than that described are given
below.
A second argorithm is as follows.
(1) A specific word, the transmission start timing of which is
fixed, is selected from the words WI.sub.1 to WI.sub.4 in one
frame.
(2) When a collision occurs between words, the delay time T.sub.i
for the preceding word is reduced by .DELTA..
(3) When an overspace occurs between words, the delay time T.sub.i
for the preceding word is increased by .DELTA..
(4) A zero inter-word time between words is not regarded as a
collision.
A third argorithm is as follows.
(1) The delay times T.sub.i for all the words WI.sub.1 to WI.sub.4
are controlled.
(2) The delay time T.sub.i is controlled to shift the word W.sub.i
in a direction for providing a space between the word WI.sub.i and
the next word WI.sub.j to avoid a collision and an overspace. For
example, in the cae of FIG. 14F, the delay time T.sub.3 for the
word WI.sub.3 is reduced while the delay time T.sub.1 for the word
WI.sub.1 is increased. Also, the delay time T.sub.1 for the word
WI.sub.4 is reduced and the delay time T.sub.1 for the word
WI.sub.4 is increased.
(3) A zero inter-word time between words is not regarded as a
collision.
According to the control argorithms mentioned above, the central
equipment detects a collision and an overspace between words and
notifies each local equipment 12.sub.i of the direction of change
of the delay time T.sub.i for the word WI.sub.i according to the
result of detection. Whenever a collision or overspace is detected,
the delay time T.sub.i is changed by a unit time.
Alternatively, the central equipment may be arranged to measure the
duration of a collision or overspace between words and
correspondingly form a timing control signal of a plurality of bits
representing the magnitude and direction of change of the delay
time, so that the control of the delay time for each word is
performed more rapidly than in the previous embodiment. Morover,
either a collision or overspace between words alone may be detected
and the delay time T.sub.i for the word WI.sub.i may be
correspondingly controlled, to permit prevention of the collision
and overspace.
* * * * *